Well, my guess is - When an aircraft crashes "conventionally" (ie, it's flying forward and hasn't broken up in the air) its high forward velocity very rapidly reduces to zero.
Therefore the forward inertia of passengers seated facing the front causes their bodies to be flung forward against their seat belt and into the back of the seat in front of them with resulting serious, if not mortal, injuries.
Whereas if they're seated facing the rear the dissipation of their forward inertial energy will be spread against their seat back, which "should" reduce injury - similar in concept to "baby capsules" used in cars.
This of course does not take into account other factors like the direction of impact, structural integrity of the seat and the airframe, fire and explosion, etc.
Besides which most pax like to face their direction of travel.
I must say that I felt better looking at the blurred scenery whizzing past the window of the Eurostar train whilst travelling at 300kph when sitting facing forward then when facing back.
Many early post WWII airliner designs proposed rearward facing seats, but like QWERTY keyboards some things are just hard to change people's mind about.
PGF

The upper deck of a C-5 and the C-141 (with seats installed) also have rear facing seats. It does feel a bit unnatural to take off facing the rear. However, a KC-10 installs its seats facing forward just like its civilian combi counterpart.

This was explained to me by an ex RAF Ground Engineer once, after I'd been onboard a VC10 and seen the seat configuration. The reason, he gave, was exactly as stated by PGF. The chances of surviving a crash with fewer injuries are greater in rearward facing seats. Purely because the inertia of the body is spread over the area of the seat rather than causing arms, legs and head to 'Flail' as would happen with forward facing seats.

As to why it isn't adopted by Civil operators, it's purely a matter of convention. In the RAF the 'passengers' sit they way there told to sit, but fare paying passengers prefer to sit facing the way their travelling.

This is not applicable to only the VC10, all RAF passenger transport aircraft have rearward facing seats.

The reason is due to biology, all your internal organs are attached to your spine. If you are facing forward in an accident the deceleration forces will tend to rip those organs from their anchorage which won't help your survival chances